TITLE: GCN CIRCULAR
NUMBER: 5761
SUBJECT: GRB 061027, Swift refined analysis of a possible burst
DATE: 06/10/27 20:02:53 GMT
FROM: Scott Barthelmy at NASA/GSFC
A. Parsons (GSFC), L. Barbier (GSFC), S.D. Barthelmy (GSFC),
J. Cummings (GSFC/UMBC), E. Fenimore (LANL), N. Gehrels (GSFC),
D. Hullinger (BYU-Idaho), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD),
D. Palmer (LANL), T. Sakamoto (GSFC/ORAU), G. Sato (GSFC/ISAS),
M. Stamatikos (GSFC/ORAU), J. Tueller (GSFC), D.N. Burrows (PSU),
V. Mangano, T. Mineo, G. Cusumano, B. Sbarufatti (INAF-IASFPA),
S.B. Pandey (UCL-MSSL)
on behalf of the Swift team:
Using the data set from T-239 to T+963 sec from recent telemetry downlinks,
we report further analysis of BAT possible GRB 061027 (trigger #235645)
(Stamatikos, et al., GCN Circ. 5760). The BAT ground-calculated position
is RA,Dec = 270.992, -82.239 deg {18h 3m 58.1s, -82d 14' 19.9"} (J2000)
+- 3.7 arcmin, (radius, sys+stat, 90% containment). The partial coding was 68%.
The event starts ~100 sec before the trigger and extends to ~T+70 sec.
All of the emission is in the 15-50 keV band -- there is nothing above 50 keV.
Because this event is very weak, we had to use 10-sec binning for the lightcurve.
There appears to be no statistically significant structure in the lightcurve.
T90 (15-350 keV) is 150 +- 20 sec (estimated error including systematics).
The time-averaged spectrum from T-100 to T+70 is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.91 +- 0.48. The fluence in the 15-150 keV band is 4.7 +- 1.5 x 10^-7 erg/cm2.
The 1-sec peak photon flux measured from T+0.00 sec in the 15-150 keV
band is 0.08 +- 0.02 ph/cm2/sec. All the quoted errors are at the 90%
confidence level.
Given the very low flux, the somewhat low detection significance, and
the soft spectrum for this event, we can not determine if this is a
long soft GRB or some hard x-ray transient from an unknown source.
The position in galactic coordinates is l, b = 311, -25, so the source
is off the plane but still possibly a galactic transient. We can not rule out
the possibility that this event is due to a chance statistical fluctuation
in the image and rate domains.
XRT observed the field of GRB061027 for 1.5 ks in PC mode starting
at 10:17:37 UT (152 seconds after the BAT trigger). No source has been
detected within the BAT error circle down to a limiting flux
of 3.6e-13 erg cm^-2 s^-1. This is unusually low for a GRB afterglow
at this point after a trigger, and suggests that this event may be
either an unusual GRB or due to some other source, either transient or noise.
UVOT began observing the field around 133 sec after the BAT trigger.
No afterglow candidate is found within the BAT error circle.
The 3-sigma upper limit of the UVOT filters are given below.
Filter T_start T_stop Exposure(s) Mag (3-sig Upper limit)
V 133 6777 805 20.41
B 735 6163 206 20.40
U 711 5958 216 20.04
W1 687 5753 235 19.96
M2 663 6839 292 19.95
W2 763 6573 216 20.29
White 152 6367 501 21.10